I'd suggest that a lot of the reason for a turtleback was tradition
Depends where you're looking. Not a lot of traditional headrests in these images:
In fact, the Gloster Gladiator was the only RAF interwar fighter that had a raised rear fuselage. Even then, it was more of an addition ontop of the Gauntlet-style fuselage to align with the cockpit canopy, rather than being a fully integrated part of the fuselage structure.